Currently the difftool --dir-diff tests may or may not use symlinks
depending on the operating system on which they are run. In one case
this has caused a test failure to be noticed only on Windows when the
test also fails on Linux when difftool is invoked with --no-symlinks.
Rewrite these tests so that they do not depend on the environment but
run explicitly with both --symlinks and --no-symlinks, protecting the
--symlinks version with a SYMLINKS prerequisite.
Signed-off-by: John Keeping <john@keeping.me.uk> Signed-off-by: Junio C Hamano <gitster@pobox.com>